Voltage or contact closure sensor

ABSTRACT

An external signal sensor uses a single sensing circuit to detect a DC voltage or an AC voltage or a contact closure. The circuit can sense different isolated signals with relatively low power consumption. According to various embodiments, an isolation transformer has a primary winding that is fed by an oscillator signal. A secondary winding of the isolation transformer is open when no contact closure or AC or DC voltage is present, but is closed when a contact closure is present or when an AC or DC voltage is present. When the secondary winding is open, a status signal at an output of the sensing circuit has a logical high value. When the secondary winding is closed, the status signal has a logical low value. In this way, the same sensing circuit can be used to detect either a contact closure or an AC or DC voltage.

TECHNICAL BACKGROUND

Control applications, such as heating, ventilation, and air conditioning(HVAC) temperature control, and control of fluid levels, often involvethe sensing of external isolated signals. Typically, the signals thatare sensed include contact closures, DC voltages, and AC 60 Hz voltageswith unknown voltage references. The signals could be connected toground or to other potentials that can be different from the referencepotential of the circuits that are attempting to evaluate their status.

FIG. 5A is a schematic diagram illustrating a conventional circuit forsensing an external direct current (DC) or alternating current (AC)voltage. FIG. 5B is a schematic diagram illustrating a conventionalcircuit for sensing an isolated contact closure. In these conventionalapproaches, different circuits are required for sensing voltages and forsensing contact closures. Requiring separate circuits to handle thesedifferent external signals adds complexity and cost to the overalldesign of a device that needs to implement both functionalities. Inaddition, requiring the sensing circuit or isolated power supply todeliver 10 milliamperes (ma) of optical diode current may be difficultin low power designs.

SUMMARY OF THE DISCLOSURE

An external signal sensor is disclosed. In particular, an externalsignal sensor is disclosed that uses a single circuit to detect a DCvoltage or an AC voltage or a contact closure. The circuit can sensedifferent isolated signals with relatively low power consumption.According to various embodiments, an isolation transformer has a primarywinding that is fed by an oscillator signal. A secondary winding of theisolation transformer is open when no contact closure or AC or DCvoltage is present, but is closed when a contact closure is present orwhen an AC or DC voltage is present. When the secondary winding is open,a status signal at an output of the sensing circuit has a logical highvalue. When the secondary winding is closed, the status signal has alogical low value. In this way, the same sensing circuit can be used todetect either a contact closure or an AC or DC voltage.

One embodiment is directed to a sensing circuit. The sensing circuitincludes an isolation transformer having a primary winding configured toreceive an oscillator signal and a secondary winding. The isolationtransformer provides a first voltage to a common node of a voltagedivider when the secondary winding is open and provides a secondvoltage, higher than the first voltage, to the common node when thesecondary winding is shorted when a contact closure is present across afirst pair of terminals. A transistor arrangement is configured to shortthe secondary winding when an alternating current (AC) or direct current(DC) signal is present across a second pair of terminals, such that thesecond voltage is provided to the common node in this scenario. Aninverter is operatively coupled to the isolation transformer andproduces a logical high output signal when the first voltage is presentat the common node. The inverter produces a logical low output signalwhen the second voltage is present at the common node.

Another embodiment is directed to a sensing circuit that includes anisolation transformer having a primary winding configured to receive anoscillator signal and a secondary winding. A voltage divider isoperatively coupled to receive the oscillator signal from the isolationtransformer and has a common node. The isolation transformer provides afirst voltage to the common node when the secondary winding is open andprovides a second voltage, higher than the first voltage, to the commonnode when the secondary winding is shorted when a contact closure ispresent across a pair of input terminals. A transistor arrangement isconfigured to short the secondary winding when an alternating current(AC) or direct current (DC) signal is present across the pair of inputterminals and to open the secondary winding when no AC or DC signal ispresent across the pair of input terminals. An inverter is operativelycoupled to the isolation transformer and produces a logical high outputsignal when the first voltage is present at the common node and alogical low output signal when the second voltage is present at thecommon node.

Yet another embodiment is directed to a sensing circuit that includes anisolation transformer having a primary winding configured to receive anoscillator signal and a secondary winding. A peak filter is operativelycoupled to receive a signal from the isolation transformer and isconfigured to generate a filtered signal. The signal has a first voltagewhen the secondary winding is open and a second voltage, higher than thefirst voltage, when the secondary winding is shorted when a contactclosure is present across a first pair of input terminals. A transistorarrangement is configured to short the secondary winding when analternating current (AC) or direct current (DC) signal is present acrossa second pair of input terminals. The transistor arrangement opens thesecondary winding when no AC or DC signal is present across the secondpair of input terminals. An inverter is operatively coupled to theisolation transformer and is configured to produce a logical high outputsignal when the signal has the first voltage and to produce a logicallow output signal when the signal has the second voltage.

Various embodiments may realize certain advantages. For example, asingle circuit can be used to sense different isolated signals, such as,for example, an AC voltage, a DC voltage, or a contact closure. Thissingle-circuit design may reduce complexity and cost relative toconventional designs. In addition, because the oscillator signal sourceneeds only a minimal current, the energy required to operate the sensingcircuit can be reduced substantially.

Other features and advantages of the described embodiments may becomeapparent from the following detailed description and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofvarious embodiments, is better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings exemplary embodiments of various aspectsof the invention; however, the invention is not limited to the specificmethods and instrumentalities disclosed. In the drawings:

FIG. 1 is a diagram of an exemplary metering system;

FIG. 2 expands upon the diagram of FIG. 1 and illustrates an exemplarymetering system in greater detail;

FIG. 3A is a block diagram illustrating an exemplary collector;

FIG. 3B is a block diagram illustrating an exemplary meter;

FIG. 4 is a diagram of an exemplary subnet of a wireless network forcollecting data from remote devices;

FIG. 5A is schematic diagram illustrating an example conventionalcircuit for sensing an external direct current (DC) or alternatingcurrent (AC) voltage;

FIG. 5B is a schematic diagram illustrating an example conventionalcircuit for sensing an isolated contact closure; and

FIG. 6 is a schematic diagram illustrating an example circuit forsensing a voltage or a contact closure according to an embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Exemplary systems and methods for gathering meter data are describedbelow with reference to FIGS. 1-6. It will be appreciated by those ofordinary skill in the art that the description given herein with respectto those figures is for exemplary purposes only and is not intended inany way to limit the scope of potential embodiments.

Generally, a plurality of meter devices, which operate to track usage ofa service or commodity such as, for example, electricity, water, andgas, are operable to wirelessly communicate. One or more devices,referred to herein as “collectors,” are provided that “collect” datatransmitted by the other meter devices so that it can be accessed byother computer systems. The collectors receive and compile metering datafrom a plurality of meter devices via wireless communications. A datacollection server may communicate with the collectors to retrieve thecompiled meter data.

FIG. 1 provides a diagram of one exemplary metering system 110. System110 comprises a plurality of meters 114, which are operable to sense andrecord consumption or usage of a service or commodity such as, forexample, electricity, water, or gas. Meters 114 may be located atcustomer premises such as, for example, a home or place of business.Meters 114 comprise circuitry for measuring the consumption of theservice or commodity being consumed at their respective locations andfor generating data reflecting the consumption, as well as other datarelated thereto. Meters 114 may also comprise circuitry for wirelesslytransmitting data generated by the meter to a remote location. Meters114 may further comprise circuitry for receiving data, commands orinstructions wirelessly as well. Meters that are operable to bothreceive and transmit data may be referred to as “bi-directional” or“two-way” meters, while meters that are only capable of transmittingdata may be referred to as “transmit-only” or “one-way” meters. Inbi-directional meters, the circuitry for transmitting and receiving maycomprise a transceiver. In an illustrative embodiment, meters 114 maybe, for example, electricity meters manufactured by Elster Electricity,LLC and marketed under the tradename REX.

System 110 further comprises collectors 116. In one embodiment,collectors 116 are also meters operable to detect and record usage of aservice or commodity such as, for example, electricity, water, or gas.In addition, collectors 116 are operable to send data to and receivedata from meters 114. Thus, like the meters 114, the collectors 116 maycomprise both circuitry for measuring the consumption of a service orcommodity and for generating data reflecting the consumption andcircuitry for transmitting and receiving data. In one embodiment,collector 116 and meters 114 communicate with and amongst one anotherusing any one of several wireless techniques such as, for example,frequency hopping spread spectrum (FHSS) and direct sequence spreadspectrum (DSSS).

A collector 116 and the meters 114 with which it communicates define asubnet/LAN 120 of system 110. As used herein, meters 114 and collectors116 may be referred to as “nodes” in the subnet 120. In each subnet/LAN120, each meter transmits data related to consumption of the commoditybeing metered at the meter's location. The collector 116 receives thedata transmitted by each meter 114, effectively “collecting” it, andthen periodically transmits the data from all of the meters in thesubnet/LAN 120 to a data collection server 206. The data collectionserver 206 stores the data for analysis and preparation of bills, forexample. The data collection server 206 may be a specially programmedgeneral purpose computing system and may communicate with collectors 116via a network 112. The network 112 may comprise any form of network,including a wireless network or a fixed-wire network, such as a localarea network (LAN), a wide area network, the Internet, an intranet, atelephone network, such as the public switched telephone network (PSTN),a Frequency Hopping Spread Spectrum (FHSS) radio network, a meshnetwork, a Wi-Fi (802.11) network, a Wi-Max (802.16) network, a landline (POTS) network, or any combination of the above.

Referring now to FIG. 2, further details of the metering system 110 areshown. Typically, the system will be operated by a utility company or acompany providing information technology services to a utility company.As shown, the system 110 comprises a network management server 202, anetwork management system (NMS) 204 and the data collection server 206that together manage one or more subnets/LANs 120 and their constituentnodes. The NMS 204 tracks changes in network state, such as new nodesregistering/unregistering with the system 110, node communication pathschanging, etc. This information is collected for each subnet/LAN 120 andis detected and forwarded to the network management server 202 and datacollection server 206.

Each of the meters 114 and collectors 116 is assigned an identifier (LANID) that uniquely identifies that meter or collector on its subnet/LAN120. In this embodiment, communication between nodes (i.e., thecollectors and meters) and the system 110 is accomplished using the LANID. However, it is preferable for operators of a utility to query andcommunicate with the nodes using their own identifiers. To this end, amarriage file 208 may be used to correlate a utility's identifier for anode (e.g., a utility serial number) with both a manufacturer serialnumber (i.e., a serial number assigned by the manufacturer of the meter)and the LAN ID for each node in the subnet/LAN 120. In this manner, theutility can refer to the meters and collectors by the utilitiesidentifier, while the system can employ the LAN ID for the purpose ofdesignating particular meters during system communications.

A device configuration database 210 stores configuration informationregarding the nodes. For example, in the metering system 200, the deviceconfiguration database may include data regarding time of use (TOU)switchpoints, etc. for the meters 114 and collectors 116 communicatingin the system 110. A data collection requirements database 212 containsinformation regarding the data to be collected on a per node basis. Forexample, a utility may specify that metering data such as load profile,demand, TOU, etc. is to be collected from particular meter(s) 114 a.Reports 214 containing information on the network configuration may beautomatically generated or in accordance with a utility request.

The network management system (NMS) 204 maintains a database describingthe current state of the global fixed network system (current networkstate 220) and a database describing the historical state of the system(historical network state 222). The current network state 220 containsdata regarding current meter-to-collector assignments, etc. for eachsubnet/LAN 120. The historical network state 222 is a database fromwhich the state of the network at a particular point in the past can bereconstructed. The NMS 204 is responsible for, amongst other things,providing reports 214 about the state of the network. The NMS 204 may beaccessed via an API 220 that is exposed to a user interface 216 and aCustomer Information System (CIS) 218. Other external interfaces mayalso be implemented. In addition, the data collection requirementsstored in the database 212 may be set via the user interface 216 or CIS218.

The data collection server 206 collects data from the nodes (e.g.,collectors 116) and stores the data in a database 224. The data includesmetering information, such as energy consumption and may be used forbilling purposes, etc. by a utility provider.

The network management server 202, network management system 204 anddata collection server 206 communicate with the nodes in each subnet/LAN120 via network 110.

FIG. 3A is a block diagram illustrating further details of oneembodiment of a collector 116. Although certain components aredesignated and discussed with reference to FIG. 3A, it should beappreciated that the invention is not limited to such components. Infact, various other components typically found in an electronic metermay be a part of collector 116, but have not been shown in FIG. 3A forthe purposes of clarity and brevity. Also, the invention may use othercomponents to accomplish the operation of collector 116. The componentsthat are shown and the functionality described for collector 116 areprovided as examples, and are not meant to be exclusive of othercomponents or other functionality.

As shown in FIG. 3A, collector 116 may comprise metering circuitry 304that performs measurement of consumption of a service or commodity and aprocessor 305 that controls the overall operation of the meteringfunctions of the collector 116. The collector 116 may further comprise adisplay 310 for displaying information such as measured quantities andmeter status and a memory 312 for storing data. The collector 116further comprises wireless LAN communications circuitry 306 forcommunicating wirelessly with the meters 114 in a subnet/LAN and anetwork interface 308 for communication over the network 112.

In one embodiment, the metering circuitry 304, processor 305, display310 and memory 312 are implemented using an A3 ALPHA meter availablefrom Elster Electricity, Inc. In that embodiment, the wireless LANcommunications circuitry 306 may be implemented by a LAN Option Board(e.g., a 900 MHz two-way radio) installed within the A3 ALPHA meter, andthe network interface 308 may be implemented by a WAN Option Board(e.g., a telephone modem) also installed within the A3 ALPHA meter. Inthis embodiment, the WAN Option Board 308 routes messages from network112 (via interface port 302) to either the meter processor 305 or theLAN Option Board 306. LAN Option Board 306 may use a transceiver (notshown), for example a 900 MHz radio, to communicate data to meters 114.Also, LAN Option Board 306 may have sufficient memory to store datareceived from meters 114. This data may include, but is not limited tothe following: current billing data (e.g., the present values stored anddisplayed by meters 114), previous billing period data, previous seasondata, and load profile data.

LAN Option Board 306 may be capable of synchronizing its time to a realtime clock (not shown) in A3 ALPHA meter, thereby synchronizing the LANreference time to the time in the meter. The processing necessary tocarry out the communication functionality and the collection and storageof metering data of the collector 116 may be handled by the processor305 and/or additional processors (not shown) in the LAN Option Board 306and the WAN Option Board 308.

The responsibility of a collector 116 is wide and varied. Generally,collector 116 is responsible for managing, processing and routing datacommunicated between the collector and network 112 and between thecollector and meters 114. Collector 116 may continually orintermittently read the current data from meters 114 and store the datain a database (not shown) in collector 116. Such current data mayinclude but is not limited to the total kWh usage, the Time-Of-Use (TOU)kWh usage, peak kW demand, and other energy consumption measurements andstatus information. Collector 116 also may read and store previousbilling and previous season data from meters 114 and store the data inthe database in collector 116. The database may be implemented as one ormore tables of data within the collector 116.

FIG. 3B is a block diagram of an exemplary embodiment of a meter 114that may operate in the system 110 of FIGS. 1 and 2. As shown, the meter114 comprises metering circuitry 304′ for measuring the amount of aservice or commodity that is consumed, a processor 305′ that controlsthe overall functions of the meter, a display 310′ for displaying meterdata and status information, and a memory 312′ for storing data andprogram instructions. The meter 114 further comprises wirelesscommunications circuitry 306′ for transmitting and receiving datato/from other meters 114 or a collector 116.

Referring again to FIG. 1, in the exemplary embodiment shown, acollector 116 directly communicates with only a subset of the pluralityof meters 114 in its particular subnet/LAN. Meters 114 with whichcollector 116 directly communicates may be referred to as “level one”meters 114 a. The level one meters 114 a are said to be one “hop” fromthe collector 116. Communications between collector 116 and meters 114other than level one meters 114 a are relayed through the level onemeters 114 a. Thus, the level one meters 114 a operate as repeaters forcommunications between collector 116 and meters 114 located further awayin subnet 120.

Each level one meter 114 a typically will only be in range to directlycommunicate with only a subset of the remaining meters 114 in the subnet120. The meters 114 with which the level one meters 114 a directlycommunicate may be referred to as level two meters 114 b. Level twometers 114 b are one “hop” from level one meters 114 a, and thereforetwo “hops” from collector 116. Level two meters 114 b operate asrepeaters for communications between the level one meters 114 a andmeters 114 located further away from collector 116 in the subnet 120.

While only three levels of meters are shown (collector 116, first level114 a, second level 114 b) in FIG. 1, a subnet 120 may comprise anynumber of levels of meters 114. For example, a subnet 120 may compriseone level of meters but might also comprise eight or more levels ofmeters 114. In an embodiment wherein a subnet comprises eight levels ofmeters 114, as many as 1024 meters might be registered with a singlecollector 116.

As mentioned above, each meter 114 and collector 116 that is installedin the system 110 has a unique identifier (LAN ID) stored thereon thatuniquely identifies the device from all other devices in the system 110.Additionally, meters 114 operating in a subnet 120 comprise informationincluding the following: data identifying the collector with which themeter is registered; the level in the subnet at which the meter islocated; the repeater meter at the prior level with which the metercommunicates to send and receive data to/from the collector; anidentifier indicating whether the meter is a repeater for other nodes inthe subnet; and if the meter operates as a repeater, the identifier thatuniquely identifies the repeater within the particular subnet, and thenumber of meters for which it is a repeater. Collectors 116 have storedthereon all of this same data for all meters 114 that are registeredtherewith. Thus, collector 116 comprises data identifying all nodesregistered therewith as well as data identifying the registered path bywhich data is communicated from the collector to each node. Each meter114 therefore has a designated communications path to the collector thatis either a direct path (e.g., all level one nodes) or an indirect paththrough one or more intermediate nodes that serve as repeaters.

Information is transmitted in this embodiment in the form of packets.For most network tasks such as, for example, reading meter data,collector 116 communicates with meters 114 in the subnet 120 usingpoint-to-point transmissions. For example, a message or instruction fromcollector 116 is routed through the designated set of repeaters to thedesired meter 114. Similarly, a meter 114 communicates with collector116 through the same set of repeaters, but in reverse.

In some instances, however, collector 116 may need to quicklycommunicate information to all meters 114 located in its subnet 120.Accordingly, collector 116 may issue a broadcast message that is meantto reach all nodes in the subnet 120. The broadcast message may bereferred to as a “flood broadcast message.” A flood broadcast originatesat collector 116 and propagates through the entire subnet 120 one levelat a time. For example, collector 116 may transmit a flood broadcast toall first level meters 114 a. The first level meters 114 a that receivethe message pick a random time slot and retransmit the broadcast messageto second level meters 114 b. Any second level meter 114 b can acceptthe broadcast, thereby providing better coverage from the collector outto the end point meters. Similarly, the second level meters 114 b thatreceive the broadcast message pick a random time slot and communicatethe broadcast message to third level meters. This process continues outuntil the end nodes of the subnet. Thus, a broadcast message graduallypropagates outward from the collector to the nodes of the subnet 120.

The flood broadcast packet header contains information to prevent nodesfrom repeating the flood broadcast packet more than once per level. Forexample, within a flood broadcast message, a field might exist thatindicates to meters/nodes which receive the message, the level of thesubnet the message is located; only nodes at that particular level mayre-broadcast the message to the next level. If the collector broadcastsa flood message with a level of 1, only level 1 nodes may respond. Priorto re-broadcasting the flood message, the level 1 nodes increment thefield to 2 so that only level 2 nodes respond to the broadcast.Information within the flood broadcast packet header ensures that aflood broadcast will eventually die out.

Generally, a collector 116 issues a flood broadcast several times, e.g.five times, successively to increase the probability that all meters inthe subnet 120 receive the broadcast. A delay is introduced before eachnew broadcast to allow the previous broadcast packet time to propagatethrough all levels of the subnet.

Meters 114 may have a clock formed therein. However, meters 114 oftenundergo power interruptions that can interfere with the operation of anyclock therein. Accordingly, the clocks internal to meters 114 cannot berelied upon to provide an accurate time reading. Having the correct timeis necessary, however, when time of use metering is being employed.Indeed, in an embodiment, time of use schedule data may also becomprised in the same broadcast message as the time. Accordingly,collector 116 periodically flood broadcasts the real time to meters 114in subnet 120. Meters 114 use the time broadcasts to stay synchronizedwith the rest of the subnet 120. In an illustrative embodiment,collector 116 broadcasts the time every 15 minutes. The broadcasts maybe made near the middle of 15 minute clock boundaries that are used inperforming load profiling and time of use (TOU) schedules so as tominimize time changes near these boundaries. Maintaining timesynchronization is important to the proper operation of the subnet 120.Accordingly, lower priority tasks performed by collector 116 may bedelayed while the time broadcasts are performed.

In an illustrative embodiment, the flood broadcasts transmitting timedata may be repeated, for example, five times, so as to increase theprobability that all nodes receive the time. Furthermore, where time ofuse schedule data is communicated in the same transmission as the timingdata, the subsequent time transmissions allow a different piece of thetime of use schedule to be transmitted to the nodes.

Exception messages are used in subnet 120 to transmit unexpected eventsthat occur at meters 114 to collector 116. In an embodiment, the first 4seconds of every 32-second period are allocated as an exception windowfor meters 114 to transmit exception messages. Meters 114 transmit theirexception messages early enough in the exception window so the messagehas time to propagate to collector 116 before the end of the exceptionwindow. Collector 116 may process the exceptions after the 4-secondexception window. Generally, a collector 116 acknowledges exceptionmessages, and collector 116 waits until the end of the exception windowto send this acknowledgement.

In an illustrative embodiment, exception messages are configured as oneof three different types of exception messages: local exceptions, whichare handled directly by the collector 116 without intervention from datacollection server 206; an immediate exception, which is generallyrelayed to data collection server 206 under an expedited schedule; and adaily exception, which is communicated to the communication server 122on a regular schedule.

Exceptions are processed as follows. When an exception is received atcollector 116, the collector 116 identifies the type of exception thathas been received. If a local exception has been received, collector 116takes an action to remedy the problem. For example, when collector 116receives an exception requesting a “node scan request” such as discussedbelow, collector 116 transmits a command to initiate a scan procedure tothe meter 114 from which the exception was received.

If an immediate exception type has been received, collector 116 makes arecord of the exception. An immediate exception might identify, forexample, that there has been a power outage. Collector 116 may log thereceipt of the exception in one or more tables or files. In anillustrative example, a record of receipt of an immediate exception ismade in a table referred to as the “Immediate Exception Log Table.”Collector 116 then waits a set period of time before taking furtheraction with respect to the immediate exception. For example, collector116 may wait 64 seconds. This delay period allows the exception to becorrected before communicating the exception to the data collectionserver 206. For example, where a power outage was the cause of theimmediate exception, collector 116 may wait a set period of time toallow for receipt of a message indicating the power outage has beencorrected.

If the exception has not been corrected, collector 116 communicates theimmediate exception to data collection server 206. For example,collector 116 may initiate a dial-up connection with data collectionserver 206 and download the exception data. After reporting an immediateexception to data collection server 206, collector 116 may delayreporting any additional immediate exceptions for a period of time suchas ten minutes. This is to avoid reporting exceptions from other meters114 that relate to, or have the same cause as, the exception that wasjust reported.

If a daily exception was received, the exception is recorded in a fileor a database table. Generally, daily exceptions are occurrences in thesubnet 120 that need to be reported to data collection server 206, butare not so urgent that they need to be communicated immediately. Forexample, when collector 116 registers a new meter 114 in subnet 120,collector 116 records a daily exception identifying that theregistration has taken place. In an illustrative embodiment, theexception is recorded in a database table referred to as the “DailyException Log Table.” Collector 116 communicates the daily exceptions todata collection server 206. Generally, collector 116 communicates thedaily exceptions once every 24 hours.

In the present embodiment, a collector assigns designated communicationspaths to meters with bi-directional communication capability, and maychange the communication paths for previously registered meters ifconditions warrant. For example, when a collector 116 is initiallybrought into system 110, it needs to identify and register meters in itssubnet 120. A “node scan” refers to a process of communication between acollector 116 and meters 114 whereby the collector may identify andregister new nodes in a subnet 120 and allow previously registered nodesto switch paths. A collector 116 can implement a node scan on the entiresubnet, referred to as a “full node scan,” or a node scan can beperformed on specially identified nodes, referred to as a “node scanretry.”

A full node scan may be performed, for example, when a collector isfirst installed. The collector 116 must identify and register nodes fromwhich it will collect usage data. The collector 116 initiates a nodescan by broadcasting a request, which may be referred to as a Node ScanProcedure request. Generally, the Node Scan Procedure request directsthat all unregistered meters 114 or nodes that receive the requestrespond to the collector 116. The request may comprise information suchas the unique address of the collector that initiated the procedure. Thesignal by which collector 116 transmits this request may have limitedstrength and therefore is detected only at meters 114 that are inproximity of collector 116. Meters 114 that receive the Node ScanProcedure request respond by transmitting their unique identifier aswell as other data.

For each meter from which the collector receives a response to the NodeScan Procedure request, the collector tries to qualify thecommunications path to that meter before registering the meter with thecollector. That is, before registering a meter, the collector 116attempts to determine whether data communications with the meter will besufficiently reliable. In one embodiment, the collector 116 determineswhether the communication path to a responding meter is sufficientlyreliable by comparing a Received Signal Strength Indication (RSSI) value(i.e., a measurement of the received radio signal strength) measuredwith respect to the received response from the meter to a selectedthreshold value. For example, the threshold value may be −60 dBm. RSSIvalues above this threshold would be deemed sufficiently reliable. Inanother embodiment, qualification is performed by transmitting apredetermined number of additional packets to the meter, such as tenpackets, and counting the number of acknowledgements received back fromthe meter. If the number of acknowledgments received is greater than orequal to a selected threshold (e.g., 8 out of 10), then the path isconsidered to be reliable. In other embodiments, a combination of thetwo qualification techniques may be employed.

If the qualification threshold is not met, the collector 116 may add anentry for the meter to a “Straggler Table.” The entry includes themeter's LAN ID, its qualification score (e.g., 5 out of 10; or its RSSIvalue), its level (in this case level one) and the unique ID of itsparent (in this case the collector's ID).

If the qualification threshold is met or exceeded, the collector 116registers the node. Registering a meter 114 comprises updating a list ofthe registered nodes at collector 116. For example, the list may beupdated to identify the meter's system-wide unique identifier and thecommunication path to the node. Collector 116 also records the meter'slevel in the subnet (i.e. whether the meter is a level one node, leveltwo node, etc.), whether the node operates as a repeater, and if so, thenumber of meters for which it operates as a repeater. The registrationprocess further comprises transmitting registration information to themeter 114. For example, collector 116 forwards to meter 114 anindication that it is registered, the unique identifier of the collectorwith which it is registered, the level the meter exists at in thesubnet, and the unique identifier of its parent meter that will serveras a repeater for messages the meter may send to the collector. In thecase of a level one node, the parent is the collector itself. The meterstores this data and begins to operate as part of the subnet byresponding to commands from its collector 116.

Qualification and registration continues for each meter that responds tothe collector's initial Node Scan Procedure request. The collector 116may rebroadcast the Node Scan Procedure additional times so as to insurethat all meters 114 that may receive the Node Scan Procedure have anopportunity for their response to be received and the meter qualified asa level one node at collector 116.

The node scan process then continues by performing a similar process asthat described above at each of the now registered level one nodes. Thisprocess results in the identification and registration of level twonodes. After the level two nodes are identified, a similar node scanprocess is performed at the level two nodes to identify level threenodes, and so on.

Specifically, to identify and register meters that will become level twometers, for each level one meter, in succession, the collector 116transmits a command to the level one meter, which may be referred to asan “Initiate Node Scan Procedure” command. This command instructs thelevel one meter to perform its own node scan process. The requestcomprises several data items that the receiving meter may use incompleting the node scan. For example, the request may comprise thenumber of timeslots available for responding nodes, the unique addressof the collector that initiated the request, and a measure of thereliability of the communications between the target node and thecollector. As described below, the measure of reliability may beemployed during a process for identifying more reliable paths forpreviously registered nodes.

The meter that receives the Initiate Node Scan Response request respondsby performing a node scan process similar to that described above. Morespecifically, the meter broadcasts a request to which all unregisterednodes may respond. The request comprises the number of timeslotsavailable for responding nodes (which is used to set the period for thenode to wait for responses), the unique address of the collector thatinitiated the node scan procedure, a measure of the reliability of thecommunications between the sending node and the collector (which may beused in the process of determining whether a meter's path may beswitched as described below), the level within the subnet of the nodesending the request, and an RSSI threshold (which may also be used inthe process of determining whether a registered meter's path may beswitched). The meter issuing the node scan request then waits for andreceives responses from unregistered nodes. For each response, the meterstores in memory the unique identifier of the responding meter. Thisinformation is then transmitted to the collector.

For each unregistered meter that responded to the node scan issued bythe level one meter, the collector attempts again to determine thereliability of the communication path to that meter. In one embodiment,the collector sends a “Qualify Nodes Procedure” command to the level onenode which instructs the level one node to transmit a predeterminednumber of additional packets to the potential level two node and torecord the number of acknowledgements received back from the potentiallevel two node. This qualification score (e.g., 8 out of 10) is thentransmitted back to the collector, which again compares the score to aqualification threshold. In other embodiments, other measures of thecommunications reliability may be provided, such as an RSSI value.

If the qualification threshold is not met, then the collector adds anentry for the node in the Straggler Table, as discussed above. However,if there already is an entry in the Straggler Table for the node, thecollector will update that entry only if the qualification score forthis node scan procedure is better than the recorded qualification scorefrom the prior node scan that resulted in an entry for the node.

If the qualification threshold is met or exceeded, the collector 116registers the node. Again, registering a meter 114 at level twocomprises updating a list of the registered nodes at collector 116. Forexample, the list may be updated to identify the meter's uniqueidentifier and the level of the meter in the subnet. Additionally, thecollector's 116 registration information is updated to reflect that themeter 114 from which the scan process was initiated is identified as arepeater (or parent) for the newly registered node. The registrationprocess further comprises transmitting information to the newlyregistered meter as well as the meter that will serve as a repeater forthe newly added node. For example, the node that issued the node scanresponse request is updated to identify that it operates as a repeaterand, if it was previously registered as a repeater, increments a dataitem identifying the number of nodes for which it serves as a repeater.Thereafter, collector 116 forwards to the newly registered meter anindication that it is registered, an identification of the collector 116with which it is registered, the level the meter exists at in thesubnet, and the unique identifier of the node that will serve as itsparent, or repeater, when it communicates with the collector 116.

The collector then performs the same qualification procedure for eachother potential level two node that responded to the level one node'snode scan request. Once that process is completed for the first levelone node, the collector initiates the same procedure at each other levelone node until the process of qualifying and registering level two nodeshas been completed at each level one node. Once the node scan procedurehas been performed by each level one node, resulting in a number oflevel two nodes being registered with the collector, the collector willthen send the Initiate Node Scan Response command to each level twonode, in turn. Each level two node will then perform the same node scanprocedure as performed by the level one nodes, potentially resulting inthe registration of a number of level three nodes. The process is thenperformed at each successive node, until a maximum number of levels isreached (e.g., seven levels) or no unregistered nodes are left in thesubnet.

It will be appreciated that in the present embodiment, during thequalification process for a given node at a given level, the collectorqualifies the last “hop” only. For example, if an unregistered noderesponds to a node scan request from a level four node, and therefore,becomes a potential level five node, the qualification score for thatnode is based on the reliability of communications between the levelfour node and the potential level five node (i.e., packets transmittedby the level four node versus acknowledgments received from thepotential level five node), not based on any measure of the reliabilityof the communications over the full path from the collector to thepotential level five node. In other embodiments, of course, thequalification score could be based on the full communication path.

At some point, each meter will have an established communication path tothe collector which will be either a direct path (i.e., level one nodes)or an indirect path through one or more intermediate nodes that serve asrepeaters. If during operation of the network, a meter registered inthis manner fails to perform adequately, it may be assigned a differentpath or possibly to a different collector as described below.

As previously mentioned, a full node scan may be performed when acollector 116 is first introduced to a network. At the conclusion of thefull node scan, a collector 116 will have registered a set of meters 114with which it communicates and reads metering data. Full node scansmight be periodically performed by an installed collector to identifynew meters 114 that have been brought on-line since the last node scanand to allow registered meters to switch to a different path.

In addition to the full node scan, collector 116 may also perform aprocess of scanning specific meters 114 in the subnet 120, which isreferred to as a “node scan retry.” For example, collector 116 may issuea specific request to a meter 114 to perform a node scan outside of afull node scan when on a previous attempt to scan the node, thecollector 116 was unable to confirm that the particular meter 114received the node scan request. Also, a collector 116 may request a nodescan retry of a meter 114 when during the course of a full node scan thecollector 116 was unable to read the node scan data from the meter 114.Similarly, a node scan retry will be performed when an exceptionprocedure requesting an immediate node scan is received from a meter114.

The system 110 also automatically reconfigures to accommodate a newmeter 114 that may be added. More particularly, the system identifiesthat the new meter has begun operating and identifies a path to acollector 116 that will become responsible for collecting the meteringdata. Specifically, the new meter will broadcast an indication that itis unregistered. In one embodiment, this broadcast might be, forexample, embedded in, or relayed as part of a request for an update ofthe real time as described above. The broadcast will be received at oneof the registered meters 114 in proximity to the meter that isattempting to register. The registered meter 114 forwards the time tothe meter that is attempting to register. The registered node alsotransmits an exception request to its collector 116 requesting that thecollector 116 implement a node scan, which presumably will locate andregister the new meter. The collector 116 then transmits a request thatthe registered node perform a node scan. The registered node willperform the node scan, during which it requests that all unregisterednodes respond. Presumably, the newly added, unregistered meter willrespond to the node scan. When it does, the collector will then attemptto qualify and then register the new node in the same manner asdescribed above.

Once a communication path between the collector and a meter isestablished, the meter can begin transmitting its meter data to thecollector and the collector can transmit data and instructions to themeter. As mentioned above, data is transmitted in packets. “Outbound”packets are packets transmitted from the collector to a meter at a givenlevel. In one embodiment, outbound packets contain the following fields,but other fields may also be included:

-   Length—the length of the packet;-   SrcAddr—source address—in this case, the ID of the collector;-   DestAddr—the LAN ID of the meter to which the packet addressed;    -   RptPath—the communication path to the destination meter (i.e.,        the list of identifiers of each repeater in the path from the        collector to the destination node); and    -   Data—the payload of the packet.        The packet may also include integrity check information (e.g.,        CRC), a pad to fill-out unused portions of the packet and other        control information. When the packet is transmitted from the        collector, it will only be forwarded on to the destination meter        by those repeater meters whose identifiers appear in the RptPath        field. Other meters that may receive the packet, but that are        not listed in the path identified in the RptPath field will not        repeat the packet.

“Inbound” packets are packets transmitted from a meter at a given levelto the collector. In one embodiment, inbound packets contain thefollowing fields, but other fields may also be included:

-   Length—the length of the packet;-   SrcAddr—source address—the address of the meter that initiated the    packet;-   DestAddr—the ID of the collector to which the packet is to be    transmitted;    -   RptAddr—the ID of the parent node that serves as the next        repeater for the sending node;    -   Data—the payload of the packet;        Because each meter knows the identifier of its parent node        (i.e., the node in the next lower level that serves as a        repeater for the present node), an inbound packet need only        identify who is the next parent. When a node receives an inbound        packet, it checks to see if the RptAddr matches its own        identifier. If not, it discards the packet. If so, it knows that        it is supposed to forward the packet on toward the collector.        The node will then replace the RptAddr field with the identifier        of its own parent and will then transmit the packet so that its        parent will receive it. This process will continue through each        repeater at each successive level until the packet reaches the        collector.

For example, suppose a meter at level three initiates transmission of apacket destined for its collector. The level three node will insert inthe RptAddr field of the inbound packet the identifier of the level twonode that serves as a repeater for the level three node. The level threenode will then transmit the packet. Several level two nodes may receivethe packet, but only the level two node having an identifier thatmatches the identifier in the RptAddr field of the packet willacknowledge it. The other will discard it. When the level two node withthe matching identifier receives the packet, it will replace the RptAddrfield of the packet with the identifier of the level one packet thatserves as a repeater for that level two packet, and the level two packetwill then transmit the packet. This time, the level one node having theidentifier that matches the RptAddr field will receive the packet. Thelevel one node will insert the identifier of the collector in theRptAddr field and will transmit the packet. The collector will thenreceive the packet to complete the transmission.

A collector 116 periodically retrieves meter data from the meters thatare registered with it. For example, meter data may be retrieved from ameter every 4 hours. Where there is a problem with reading the meterdata on the regularly scheduled interval, the collector will try to readthe data again before the next regularly scheduled interval.Nevertheless, there may be instances wherein the collector 116 is unableto read metering data from a particular meter 114 for a prolonged periodof time. The meters 114 store an indication of when they are read bytheir collector 116 and keep track of the time since their data has lastbeen collected by the collector 116. If the length of time since thelast reading exceeds a defined threshold, such as for example, 18 hours,presumably a problem has arisen in the communication path between theparticular meter 114 and the collector 116. Accordingly, the meter 114changes its status to that of an unregistered meter and attempts tolocate a new path to a collector 116 via the process described above fora new node. Thus, the exemplary system is operable to reconfigure itselfto address inadequacies in the system.

In some instances, while a collector 116 may be able to retrieve datafrom a registered meter 114 occasionally, the level of success inreading the meter may be inadequate. For example, if a collector 116attempts to read meter data from a meter 114 every 4 hours but is ableto read the data, for example, only 70 percent of the time or less, itmay be desirable to find a more reliable path for reading the data fromthat particular meter. Where the frequency of reading data from a meter114 falls below a desired success level, the collector 116 transmits amessage to the meter 114 to respond to node scans going forward. Themeter 114 remains registered but will respond to node scans in the samemanner as an unregistered node as described above. In other embodiments,all registered meters may be permitted to respond to node scans, but ameter will only respond to a node scan if the path to the collectorthrough the meter that issued the node scan is shorter (i.e., less hops)than the meter's current path to the collector. A lesser number of hopsis assumed to provide a more reliable communication path than a longerpath. A node scan request always identifies the level of the node thattransmits the request, and using that information, an already registerednode that is permitted to respond to node scans can determine if apotential new path to the collector through the node that issued thenode scan is shorter than the node's current path to the collector.

If an already registered meter 114 responds to a node scan procedure,the collector 116 recognizes the response as originating from aregistered meter but that by re-registering the meter with the node thatissued the node scan, the collector may be able to switch the meter to anew, more reliable path. The collector 116 may verify that the RSSIvalue of the node scan response exceeds an established threshold. If itdoes not, the potential new path will be rejected. However, if the RSSIthreshold is met, the collector 116 will request that the node thatissued the node scan perform the qualification process described above(i.e., send a predetermined number of packets to the node and count thenumber of acknowledgements received). If the resulting qualificationscore satisfies a threshold, then the collector will register the nodewith the new path. The registration process comprises updating thecollector 116 and meter 114 with data identifying the new repeater (i.e.the node that issued the node scan) with which the updated node will nowcommunicate. Additionally, if the repeater has not previously performedthe operation of a repeater, the repeater would need to be updated toidentify that it is a repeater. Likewise, the repeater with which themeter previously communicated is updated to identify that it is nolonger a repeater for the particular meter 114. In other embodiments,the threshold determination with respect to the RSSI value may beomitted. In such embodiments, only the qualification of the last “hop”(i.e., sending a predetermined number of packets to the node andcounting the number of acknowledgements received) will be performed todetermine whether to accept or reject the new path.

In some instances, a more reliable communication path for a meter mayexist through a collector other than that with which the meter isregistered. A meter may automatically recognize the existence of themore reliable communication path, switch collectors, and notify theprevious collector that the change has taken place. The process ofswitching the registration of a meter from a first collector to a secondcollector begins when a registered meter 114 receives a node scanrequest from a collector 116 other than the one with which the meter ispresently registered. Typically, a registered meter 114 does not respondto node scan requests. However, if the request is likely to result in amore reliable transmission path, even a registered meter may respond.Accordingly, the meter determines if the new collector offers apotentially more reliable transmission path. For example, the meter 114may determine if the path to the potential new collector 116 comprisesfewer hops than the path to the collector with which the meter isregistered. If not, the path may not be more reliable and the meter 114will not respond to the node scan. The meter 114 might also determine ifthe RSSI of the node scan packet exceeds an RSSI threshold identified inthe node scan information. If so, the new collector may offer a morereliable transmission path for meter data. If not, the transmission pathmay not be acceptable and the meter may not respond. Additionally, ifthe reliability of communication between the potential new collector andthe repeater that would service the meter meets a threshold establishedwhen the repeater was registered with its existing collector, thecommunication path to the new collector may be more reliable. If thereliability does not exceed this threshold, however, the meter 114 doesnot respond to the node scan.

If it is determined that the path to the new collector may be betterthan the path to its existing collector, the meter 114 responds to thenode scan. Included in the response is information regarding any nodesfor which the particular meter may operate as a repeater. For example,the response might identify the number of nodes for which the meterserves as a repeater.

The collector 116 then determines if it has the capacity to service themeter and any meters for which it operates as a repeater. If not, thecollector 116 does not respond to the meter that is attempting to changecollectors. If, however, the collector 116 determines that it hascapacity to service the meter 114, the collector 116 stores registrationinformation about the meter 114. The collector 116 then transmits aregistration command to meter 114. The meter 114 updates itsregistration data to identify that it is now registered with the newcollector. The collector 116 then communicates instructions to the meter114 to initiate a node scan request. Nodes that are unregistered, orthat had previously used meter 114 as a repeater respond to the requestto identify themselves to collector 116. The collector registers thesenodes as is described above in connection with registering newmeters/nodes.

Under some circumstances it may be necessary to change a collector. Forexample, a collector may be malfunctioning and need to be takenoff-line. Accordingly, a new communication path must be provided forcollecting meter data from the meters serviced by the particularcollector. The process of replacing a collector is performed bybroadcasting a message to unregister, usually from a replacementcollector, to all of the meters that are registered with the collectorthat is being removed from service. In one embodiment, registered metersmay be programmed to only respond to commands from the collector withwhich they are registered. Accordingly, the command to unregister maycomprise the unique identifier of the collector that is being replaced.In response to the command to unregister, the meters begin to operate asunregistered meters and respond to node scan requests. To allow theunregistered command to propagate through the subnet, when a nodereceives the command it will not unregister immediately, but ratherremain registered for a defined period, which may be referred to as the“Time to Live”. During this time to live period, the nodes continue torespond to application layer and immediate retries allowing theunregistration command to propagate to all nodes in the subnet.Ultimately, the meters register with the replacement collector using theprocedure described above.

One of collector's 116 main responsibilities within subnet 120 is toretrieve metering data from meters 114. In one embodiment, collector 116has as a goal to obtain at least one successful read of the meteringdata per day from each node in its subnet. Collector 116 attempts toretrieve the data from all nodes in its subnet 120 at a configurableperiodicity. For example, collector 116 may be configured to attempt toretrieve metering data from meters 114 in its subnet 120 once every 4hours. In greater detail, in one embodiment, the data collection processbegins with the collector 116 identifying one of the meters 114 in itssubnet 120. For example, collector 116 may review a list of registerednodes and identify one for reading. The collector 116 then communicatesa command to the particular meter 114 that it forward its metering datato the collector 116. If the meter reading is successful and the data isreceived at collector 116, the collector 116 determines if there areother meters that have not been read during the present reading session.If so, processing continues. However, if all of the meters 114 in subnet120 have been read, the collector waits a defined length of time, suchas, for example, 4 hours, before attempting another read.

If during a read of a particular meter, the meter data is not receivedat collector 116, the collector 116 begins a retry procedure wherein itattempts to retry the data read from the particular meter. Collector 116continues to attempt to read the data from the node until either thedata is read or the next subnet reading takes place. In an embodiment,collector 116 attempts to read the data every 60 minutes. Thus, whereina subnet reading is taken every 4 hours, collector 116 may issue threeretries between subnet readings.

Meters 114 are often two-way meters—i.e. they are operable to bothreceive and transmit data. However, one-way meters that are operableonly to transmit and not receive data may also be deployed. FIG. 4 is ablock diagram illustrating a subnet 401 that includes a number ofone-way meters 451-456. As shown, meters 114 a-k are two-way devices. Inthis example, the two-way meters 114 a-k operate in the exemplary mannerdescribed above, such that each meter has a communication path to thecollector 116 that is either a direct path (e.g., meters 114 a and 114 bhave a direct path to the collector 116) or an indirect path through oneor more intermediate meters that serve as repeaters. For example, meter114 h has a path to the collector through, in sequence, intermediatemeters 114 d and 114 b. In this example embodiment, when a one-way meter(e.g., meter 451) broadcasts its usage data, the data may be received atone or more two-way meters that are in proximity to the one-way meter(e.g., two-way meters 114 f and 114 g). In one embodiment, the data fromthe one-way meter is stored in each two-way meter that receives it, andthe data is designated in those two-way meters as having been receivedfrom the one-way meter. At some point, the data from the one-way meteris communicated, by each two-way meter that received it, to thecollector 116. For example, when the collector reads the two-way meterdata, it recognizes the existence of meter data from the one-way meterand reads it as well. After the data from the one-way meter has beenread, it is removed from memory.

While the collection of data from one-way meters by the collector hasbeen described above in the context of a network of two-way meters 114that operate in the manner described in connection with the embodimentsdescribed above, it is understood that the present invention is notlimited to the particular form of network established and utilized bythe meters 114 to transmit data to the collector. Rather, the presentinvention may be used in the context of any network topology in which aplurality of two-way communication nodes are capable of transmittingdata and of having that data propagated through the network of nodes tothe collector.

Control applications, such as heating, ventilation, and air conditioning(HVAC) temperature control, and control of fluid levels, often involvethe sensing of external isolated signals. Communication nodes in awireless mesh network may also involve sensing external isolatedsignals, such as an AC or DC voltage or a contact closure. The signalsthat are sensed may have unknown voltage references. The signals couldbe connected to ground or to other potentials that can be different fromthe reference potential of the circuits that are attempting to evaluatetheir status.

FIG. 5A is a schematic diagram illustrating a conventional circuit 500for sensing an external direct current (DC) or alternating current (AC)voltage. A diode bridge 502 formed by diodes 504, 506, 508, and 510allows any polarity of voltage to be sensed as long as the externalvoltage has sufficient current capacity to drive an optical diode 512.The required current capacity is typically approximately 10 milliamperes(ma) to guarantee good saturation of an optical transistor 514. For ACapplications, the zero crossover voltage will cause the optical diode512 to lose the drive current, so the collector of the opticaltransistor 514 would need to be filtered to produce a constant statussignal to indicate the presence of an external voltage.

FIG. 5B is a schematic diagram illustrating a conventional circuit 550for sensing an isolated contact closure. A contact closure draws currentfrom an isolated power supply 552. This current can then be fed into anoptical diode 554 for sensing. An optical transistor 556 on thereference side reflects the closed or open status of the contacts.

In these conventional approaches, different circuits are required forsensing voltages and for sensing contact closures. Requiring separatecircuits to handle these different external signals adds complexity andcost to the overall design of a device that needs to implement bothfunctionalities. In addition, requiring the sensing circuit or isolatedpower supply to deliver 10 milliamperes (ma) of optical diode currentmay be difficult in low power designs.

According to various example embodiments disclosed herein, a singlecircuit is used either for sensing an AC or DC voltage or for sensingcontact closure. The circuit can sense different isolated signals withlow power consumption.

FIG. 6 illustrates an example sensing circuit 600 for sensing an AC orDC voltage or a contact closure according to an embodiment. The sensingcircuit is fed by a 100 kHz oscillator signal at an input 602. Theoscillator signal can be generated from a free running oscillator (notshown), such as a 555 timer. Alternatively, the oscillator signal can begenerated using an oscillator function in a microprocessor (not shown).The signal source only needs to output a small current, e.g., a fewhundred microamperes. The oscillator signal is fed through a DCisolation capacitor 604 to the primary winding of a small isolationtransformer 606, which may be implemented as a low cost device, such asa common mode choke, as long as the transformer 606 provides sufficientcircuit isolation. The oscillator signal is then fed to a voltagedivider 608 formed by resistors 610 and 612. The capacitor 604eliminates any DC flux from the primary winding of the transformer 606.The primary inductance of the transformer 606 is relatively high, e.g.,tens of millihenries (mH). This inductance is sufficient to limit thecurrent into the voltage divider 608 to approximately 100 microampereswhen the secondary winding of the transformer 606 is open-circuited.

When the secondary winding of the transformer 606 is open, the voltageat a common node 614 of resistors 610 and 612 and of a capacitor 616 isapproximately 1.65 V with about 200 mV of peak-to-peak, 100 kHz ripple.The capacitor 616 helps to filter any fast edges that might occur due tothe inter-winding capacitance of the transformer 606. The voltage at thecommon node 614 is fed through a diode 618 to a peak filter 620 formedby resistors 622 and 624 and a capacitor 626. The filtered voltage isbelow the input threshold of an inverter 628, so the status signal atthe output of the inverter 628 has a logical high value.

When the secondary winding of the transformer 606 is shorted, areflected low impedance is present on the primary winding of thetransformer 606. As a result, the 100 kHz oscillator signal is presentat the common node 614. This signal swings between the 3.3 V of a powersupply 630 and ground. The peak filter 620 filters this 3.3 Vpeak-to-peak, 100 kHz signal to deliver a substantially constant DCvoltage to the inverter 628 that is above the logic threshold.Accordingly, the status signal has a logical low value to indicate thatthe transformer 606 is shorted. The resistor 622 reduces the loading onthe 100 kHz source due to the capacitor 626.

In this way, the sensing circuit 600 can be used to detect a contactclosure across terminals 632 and 634. Specifically, when there is nocontact closure across terminals 632 and 634, the status signal at theoutput of the inverter 628 is high. On the other hand, when there is acontact closure across terminals 632 and 634, the status signal is low.

The sensing circuit 600 can also be used to detect an AC or DC voltageacross terminals 636 and 638. The circuit on the secondary winding ofthe transformer 606 includes a transistor arrangement comprising a pairof MOSFET transistors 640 and 642, which may be implemented as n-channelMOSFET devices, and associated components. When a DC voltage of eitherpolarity is applied across the terminals 636 and 638, both transistors640 and 642 are driven on. Those of ordinary skill in the art willappreciate that n-channel MOSFET devices conduct current from the sourceto the drain when a positive gate-to-source voltage is present above theturn on threshold. An applied DC voltage across the terminals 636 and638 causes the transistors 640 and 642 to short the secondary winding ofthe transformer 606. As discussed above, when the secondary winding ofthe transformer 606 is shorted, the status signal at the output of theinverter 628 is low. Resistors 644 and 646 limit the current drawn fromthe external DC source and help protect against transient voltagesacross the terminals 636 and 638.

When an AC voltage, e.g., a 60 Hz external voltage, is applied acrossthe terminals 636 and 638, the transistors 640 and 642 are turned on dueto a positive gate-to-source voltage exceeding the turn on threshold. Acapacitor 648 maintains a positive gate-to-source voltage during thezero crossover component of the AC voltage. As with a DC voltage, whenan AC voltage is present across the terminals 636 and 638, the secondarywinding of the transformer 606 is shorted, and the resulting lowimpedance is reflected to the sensing circuit so that the status signalat the output of the inverter 628 is low. Resistors 644 and 646 limitthe current drawn from the AC input signal.

Accordingly, the sensing circuit 600 can be used to sense the presenceof an AC or DC voltage. When an AC or DC voltage is present acrossterminals 636 and 638, the status signal at the output of the inverter628 is low. On the other hand, when no AC or DC voltage is present, thestatus signal is high.

FIG. 6 depicts separate terminals for sensing contact closure (terminals632 and 634) and for sensing an AC or DC voltage (terminals 636 and638). In some embodiments, a common set of terminals, e.g., terminals636 and 638, can be used for both functions by jumpering resistors 644and 646 together when it is desired to sense contact closure.

Some embodiments may incorporate a protection device, such as atransient voltage suppressor (TVS) diode 648, if extreme transientvoltages may be present across the terminals 636 and 638 or across theterminals 632 and 634.

The disclosed embodiments may realize certain advantages overconventional arrangements in which separate circuits are used forsensing voltages and for sensing contact closures. For example, a singlecircuit can be used to sense different isolated signals, such as, forexample, an AC voltage, a DC voltage, or a contact closure. Thissingle-circuit design may reduce complexity and cost relative toconventional designs. In addition, because the oscillator signal sourceneeds only a minimal current, the energy required to operate the sensingcircuit can be reduced to approximately 3 mW. Further, unlike theconventional circuit 550, the sensing circuit 600 does not require anisolated DC power supply.

While systems and methods have been described and illustrated withreference to specific embodiments, those skilled in the art willrecognize that modification and variations may be made without departingfrom the principles described above and set forth in the followingclaims. Accordingly, reference should be made to the following claims asdescribing the scope of the present invention.

What is claimed is:
 1. A sensing circuit comprising: an isolation transformer having a primary winding configured to receive an oscillator signal and a secondary winding, the isolation transformer configured to provide a first voltage to a common node of a voltage divider when the secondary winding is open and to provide a second voltage, higher than the first voltage, to the common node when the secondary winding is shorted when a contact closure is present across a first pair of terminals; a transistor arrangement configured to short the secondary winding when an alternating current (AC) or direct current (DC) signal is present across a second pair of terminals such that the second voltage is provided to the common node; and an inverter operatively coupled to the isolation transformer and configured to produce a logical high output signal when the first voltage is present at the common node and to produce a logical low output signal when the second voltage is present at the common node.
 2. The sensing circuit of claim 1, further comprising an oscillator operatively coupled to the primary winding and configured to generate the oscillator signal.
 3. The sensing circuit of claim 1, further comprising a microprocessor operatively coupled to the primary winding and configured to execute an oscillator function to generate the oscillator signal.
 4. The sensing circuit of claim 1, wherein the isolation transformer comprises a common mode choke.
 5. The sensing circuit of claim 1, further comprising a peak filter operatively coupled to receive a signal from the isolation transformer and configured to provide a filtered signal to an input of the inverter.
 6. The sensing circuit of claim 1, wherein the transistor arrangement comprises a plurality of n-channel MOSFET devices.
 7. The sensing circuit of claim 1, further comprising a transient voltage suppressor (TVS) diode operatively coupled to the secondary winding.
 8. A sensing circuit comprising: an isolation transformer having a primary winding configured to receive an oscillator signal and a secondary winding; a voltage divider operatively coupled to receive the oscillator signal from the isolation transformer and having a common node, the isolation transformer configured to provide a first voltage to the common node when the secondary winding is open and to provide a second voltage, higher than the first voltage, to the common node when the secondary winding is shorted when a contact closure is present across a pair of input terminals; a transistor arrangement configured to short the secondary winding when an alternating current (AC) or direct current (DC) signal is present across the pair of input terminals and to open the secondary winding when no AC or DC signal is present across the pair of input terminals; and an inverter operatively coupled to the isolation transformer and configured to produce a logical high output signal when the first voltage is present at the common node and to produce a logical low output signal when the second voltage is present at the common node.
 9. The sensing circuit of claim 8, further comprising an oscillator operatively coupled to the primary winding and configured to generate the oscillator signal, the oscillator comprising one of a 555 timer and a microprocessor configured to execute an oscillator function.
 10. The sensing circuit of claim 8, further comprising a peak filter operatively coupled to receive a signal from the isolation transformer and configured to provide a filtered signal to an input of the inverter.
 11. The sensing circuit of claim 8, the isolation transformer comprising a common mode choke.
 12. The sensing circuit of claim 8, wherein the transistor arrangement comprises a plurality of n-channel MOSFET devices.
 13. The sensing circuit of claim 8, further comprising a protection device operatively coupled to the secondary winding.
 14. The sensing circuit of claim 13, wherein the protection device comprises a transient voltage suppressor (TVS) diode.
 15. A sensing circuit comprising: an isolation transformer having a primary winding configured to receive an oscillator signal and a secondary winding; a peak filter operatively coupled to receive a signal from the isolation transformer and configured to generate a filtered signal, the signal having a first voltage when the secondary winding is open and having a second voltage, higher than the first voltage, when the secondary winding is shorted when a contact closure is present across a first pair of input terminals; a transistor arrangement configured to short the secondary winding when an alternating current (AC) or direct current (DC) signal is present across a second pair of input terminals and to open the secondary winding when no AC or DC signal is present across the second pair of input terminals; and an inverter operatively coupled to the isolation transformer and configured to produce a logical high output signal when the signal has the first voltage and to produce a logical low output signal when the signal has the second voltage.
 16. The sensing circuit of claim 15, further comprising an oscillator operatively coupled to the primary winding and configured to generate the oscillator signal, the oscillator comprising one of a 555 timer and a microprocessor configured to execute an oscillator function.
 17. The sensing circuit of claim 15, the isolation transformer comprising a common mode choke.
 18. The sensing circuit of claim 15, wherein the transistor arrangement comprises a plurality of n-channel MOSFET devices.
 19. The sensing circuit of claim 15, further comprising a protection device operatively coupled to the secondary winding.
 20. The sensing circuit of claim 19, wherein the protection device comprises a transient voltage suppressor (TVS) diode. 